Infrared absorption (IR) is the state of the art method for measuring exhaled carbon dioxide (CO2) concentration and has been in clinical use for more than 30 years. It is fast responding, selective and stable and is used routinely by monitors in the operating theatre and in postoperative care. The exhaled CO2 concentration curve (called capnograph) has also been used together with respirator treatment and for more sophisticated diagnostics for instance of lung function. Together with information about the exhaled gas flow the capnograph can be used to calculate the amount of CO2 issued from the lung and this information allows the assessment of metabolic and cardiopulmonary conditions.
The IR technology is inherently complex and bulky with advanced optical and electronic components, but in recent times compact models have been developed that are portable and can also be used in emergency situations.
A different method of detecting CO2 is based on durable, rapid and reversible colorimetric detectors that change color with the concentration of the CO2. The method is presented in A Gedeon, P Krill and C Mebius: A new colorimetric breath indicator (Colibri), Anaesthesia 1994 (49) 798.
This is inherently a less complex technology but nevertheless an opto-electric system is required to produce a signal that represents the color of the sensor surface. This technique is less accurate than the IR system and does not fully meet the requirements of a modern monitor for clinical use.
However, recently it has been shown that capnography could play an important role in the home environment for improving the quality of life of people suffering from several common diseases that are associated with disturbed breathing patterns, such as asthma and different anxiety disorders. This is discussed for instance in the following publications.
T Ritz, A E Meuret, F H Wilhem, W T Roth: Changes in pCO2, Symptoms and Lung Function of Asthma Patients During Capnometry-assisted Breathing Training. Appl Psychophysiol Biofeedback 2009 (34) 1
A E Meuret, F H Wilhem, T Ritz, W T Roth: Feedback of end-tidal pCO2 as a therapeutic approach for panic disorder. Journal of Psychiatric Research 2008 (42) 560
For home use to be feasible, the entire capnograph system must be compact, very simple to use, and at least an order of magnitude less expensive than the most inexpensive IR-based units available today. Furthermore the system must be able to continuously give feedback to the user about the results obtained and provide directions on the proper actions to be taken during breathing exercise to further improve the condition of the user. It is also a desirable feature to be able to store the results for easy transmission to an external party at a distant location.
The inventors have recognized that several important functionalities are potentially available in standard so-called smartphones. Smartphones have been suggested as means of obtaining metabolic data to help tailor nutrition therapy in various disease states, which is described in US-2013/0150746 that relates to a metabolic analyser that measures exhaled oxygen and carbon dioxide to implement a method for weight and/or fitness management. In one embodiment of the metabolic analyser a built-in camera in a cellular telephone is used to detect color changes of sensing materials when breath flow passes in order to measure the oxygen and carbon dioxide levels. Specifically, the sensing material is in the shape of an assembly of nanoparticles forming a porous membrane sensor.
The present invention addresses all the above requirements of home applications of capnography. In particular, the object of the present invention is to provide a most convenient, simple and inexpensive way to perform so called capnography-assisted breath training as for instance described in the above cited medical publications.